bool isSameTree(struct TreeNode* p, struct TreeNode* q) {
    if (p == NULL && q == NULL)
        return true;
    if (p == NULL || q == NULL)
        return false;
    if (p->val != q->val)
        return false;

    return isSameTree(p->left, q->left) && isSameTree(p->right, q->right);
}